Fuchsia test runner: fixup script

This commit fixes several issues in the fuchsia-test-runner.py script:

1. Migrate from `pm` to `ffx` for package management, as `pm` is now
deprecated. Furthermore, the `pm` calls used in this script no longer
work at Fuchsia's HEAD. This is the largest change in this commit, and
impacts all steps around repository management (creation and
registration of the repo, as well as package publishing).

2. Allow for `libtest` to be either statically or dynamically linked.
The script assumed it was dynamically linked, but the current Rust
behavior at HEAD is to statically link it.

3. Minor cleanup to use `ffx --machine json` rather than string parsing.

4. Minor cleanup to the docs around the script.

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